25mph average is FAST. At that speed, wind resistance completely dominates. Drag is a function of speed squared, so a marginal imcrease in speed causes a big increase in drag. Eg if you increase speed by 10% drag goes up by 20%. To increase your performance by 20% is a huge jump if you are already near your maximum. I would say impossible.
The only real way to get significantly faster is go aero. Install aero bars and tuck down for an hour.
It's even worse than that as power need to maintain speed goes like speed cubed. So 10% increase needs 30% more power. Clip on aero bars aren't a crazy idea...